The direct carbothermic reduction of alumina has been described in U.S. Pat. Nos. 2,974,032 (Grunert et al.) and 6,440,193 B1 (Johansen et al.). It has long been recognized that the overall reaction.Al2O3+3C=2Al+3CO  (1) takes place, or can be made to take place, generally in steps such as:2Al2O3+9C=Al4C3+6CO (vapor)  (2) Al4C3+Al2O3=6Al+3CO (vapor)  (3) Al2O3+2C=Al2O+2CO (vapor)  (4) Al2O3+4Al=3Al2O (vapor)  (5), and Al=Al (vapor)  (6). 
Reaction (2) takes place at temperatures below 2000° C. and generally between 1900° C. and 2000° C. Reaction (3), which is the aluminum producing reaction, takes place at higher temperatures of about 2050° C. Very importantly, in addition to the species stated in reactions (2) and (3), volatile species including gaseous Al, reaction (6), and gaseous aluminum suboxide that is Al2O, are formed in reaction (4) or (5) and are carried away with the off gas. Unless recovered, these volatile species will represent a loss in the yield of aluminum and the large amount of energy associated with the reduction and vaporization steps.
In the overall carbothermic reduction process, the Al2O and Al gases are recovered by reacting them with carbon in a separate reactor usually called the vapor recovery unit or vapor recovery reactor, as taught, for example, in U.S. Pat. No. 6,530,970 B2 (Lindstad). There, a carbon-hydrogen gas, such as methane, butane, acetylene or the like was cracked to provide a finely dispersed carbon which could be deposited on carbon seed particles. This required an extra cracking step. Depending on the gas composition and reaction temperature, the reaction product with carbon may be Al4C3, an (Al4C3—Al2O3) liquid slag or the gases may simply condense as Al2O3. It is desirable to form Al4C3 because it is required in the smelting furnace and the energy associated with the Al and Al2O is recovered as valuable chemical energy and can be returned to the smelter. If the Al2O3 and carbon form by condensation the energy is released as heat in the vapor recovery reactor and very little is returned to the process. If an Al4C3—Al2O3 slag forms, all of the Al and some of the energy are recovered. However, the liquid product may cause bridging of the particles in the reactor making it difficult to operate the vapor recovery reactor.
Other patents relating to carbothermic reduction to produce aluminum include U.S. Pat. Nos. 4,486,229 and 4,491,472 (Troup et al. and Stevenson et al.) Dual reaction zones are described in U.S. Pat. No. 4,099,959 (Dewing et al.), where off gases are passed through granular carbon material and countercurrent to fresh coal or “green” coke in a gas scrubber. In Dewing et al., U.S. Pat. No. 4,261,736, the off-gas, containing Al vapor and Al2O is contacted with particulate carbon in a fluidized bed maintained at a temperature above the temperature at which sticky aluminum oxycarbide forms and where heated carbon enriched with Al4C3 is removed from the fluidized bed. When using carbon particles as taught in this patent the surface area of each carbon particle may eventually become covered by reaction products and the reaction rate will thus be reduced as the gas must penetrate the reaction product layer on each carbon particle in order to continue the reaction. Only a part of the carbon in the carbon particles will thus be reacted to Al4C3. Consequently, the efficiency of the reaction is low. Also Al and Al2O vapors flow higher up the reactor forming slag or condensing, and unreacted carbon enters the main smelter, which is undesirable.
In Canadian Patent No. 1,15,435 (Sood et al.) a packed carbon bed consisting of “active” carbon in whole or in part converts Al and Al2O fume gas directly into Al4C3. There “active” carbon is considered to be any form of carbon possessing a large specific surface area and consequently a relatively low strength, so that the resulting Al4C3 reaction product does not adhere strongly to the carbon particles and/or is very porous and open, so that the deposition of the reaction product does not result in cementing of the carbon particles to one another.
The variety of wood species that can be used to make charcoal is widely varied, as described in FAO FORESTRY PAPER 41 (1987) “Simple Technologies for Charcoal Making” http://www.fao.org/docre/x5328e/x5328e00.htm, Aug. 19, 2002, and includes: Dakama, Wallaba, Kautaballi, Tropical hardwood, Oak, Coconut, and Eucalyptus Saligna. U.S. Pat. No. 6,124,028 (Nagle) also discuss activated carbons and charcoal and carbonized/carbon-polymer wood products, mentioning use of Lignum, Maple, Oak, Basswood, Pine, Redwood, Balsa, and Poplar. The wood products are described as useful for furniture, brake shoes, sports equipment, tubing, brake rotors and the like.
In the carbothermic process, the type carbon used will be important to improve Al4C3 formation and decrease Al2O3 formation in the vapor recovery reactor. Reaction rates and thermodynamic considerations are important. What is needed is an efficient method for recovering the volatile Al species, and to reduce the 25% energy loss and the 25% aluminum lost leaving as a gas. Therefore, it is one of the main objects of this invention to provide a more cost and energy effective improved aluminum production process by use of new or vastly improved material in the off-gas reactor.